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+# Security restrictions for TLD
+
+Nomulus provides several security features that allow registries to impose
+additional restrictions on which domains are allowed on a TLD and which
+registrant/nameservers they can have. The restrictions can be applied to an
+entire TLD or on a per-domain basis. This document outlines the various
+restrictions available, their intended use case, and how to use them.
+
+## TLD-wide restrictions
+
+Nomulus allows registry administrators to set registrant contact and nameserver
+restrictions on a TLD. Use the following command to set the restrictions when
+creating or updating a TLD:
+
+```shell
+$ nomulus -e production [create_tld | update_tld] \
+    --allowed_nameservers {NS1},{NS2},... \
+    --allowed_registrants {REG1},{REG2},... \
+    {TLD1} {TLD2} ...
+```
+
+When nameserver or registrant restrictions are set on a TLD, any domain mutation
+flow under that TLD will verify that the supplied nameservers or registrants
+constitute a subset of the allowed nameservers or registrants on the TLD. Note
+that an empty set is not considered a legitimate subset, and consequently when
+the restrictions are set for one property, you **must** provide at least one
+corresponding value that is allowed. If no restrictions are set, it is allowed
+to create/update domains that do not have nameservers on it. Registrant, on the
+other hand, is always required.
+
+## Per-domain nameserver restrictions
+
+Registries can also elect to impose per-domain nameserver restrictions in
+Nomulus. This restriction is orthogonal to the TLD-wide nameserver restriction
+detailed above, and the allowed nameserver list can be set to an arbitrary list
+that is not related to the TLD-wide allowed nameservers list (if any). Any
+domain mutation must pass both validations (if applicable). In practice, it is
+always recommended to maintain consistency between the two types of lists, by
+making the per-domain allowed nameserver list a subset of the TLD-wide one,
+because any nameservers that are not included in both lists are effectively
+disallowed.
+
+The per-domain allowed nameserver lists are maintained in reserved lists with
+reservation type `NAMESERVER_RESTRICTED`, using a csv format, which nameservers
+delimited by colons. The following example has two allowed nameservers:
+
+```
+internaldomain,NAMESERVER_RESTRICTED,ns1.internal.tld:ns1.internal.tld
+```
+
+Reserved lists can also prohibit domain registrations for reasons other than
+nameserver restrictions. For more details on reserved lists and how they work,
+refer to the doc [here](reserved-list-management.md). Note that multiple
+reserved lists can be applied to one TLD, and if they happen to contain
+nameserver restrictions to the same label, the resulting restriction set is the
+intersection of the all allowed nameserver lists for that label.
+
+## Domain create restriction on closed TLDs
+
+Nomulus offers the ability to "lock-down" a TLD so that domain registration is
+by default forbidden unless the domain is whitelisted. The typical use case for
+this feature is for a closed TLD that wants to enforce greater security by only
+allowing registration of domains that are explicitly allowed. Such restriction
+on domain creation is achieved by setting the TLD to be "domain create
+restricted". The allowed list of domains are read from reserved lists applied on
+the TLD, with `NAMESERVER_RESTRICTED` reservation. This means that each domain
+will also need to have explicitly allowed nameservers configured in its reserved
+list entry, and the per-domain nameserver validation is performed in related
+flows.
+
+To apply domain create restriction when creating/updating a TLD:
+
+```shell
+$ nomulus -e production [create_tld | update_tld] \
+    --domain_create_restricted {TLD1} {TLD2} ...
+```
+
+Note that you do **not** have to set a TLD-wide allowed nameservers list,
+because it operates independently from the per-domain nameservers restriction
+that `NAMESERVER_RESTRICTED` reservation invokes.
+
+In addition to disabling registration of non-whitelisted domains, setting a TLD
+as domain create restricted also applies `SERVER_UPDATE_PROHIBITED` and
+`SERVER_TRANSFER_PROHIBITED` status to domains automatically when they are
+created. Therefore any domains created under such a TLD is virtually immutable.
+For more information on EPP status codes, see
+[here](https://tools.ietf.org/html/rfc5731#section-2.3).
+
+The consequence of applying these status code is that no registrar can send
+request to modify an existing domain without the registry explicitly allowing it
+on a case-by-case basis. If a change does need to be made, the registry must
+explicitly remove the status, make changes, and then reapply the status. To
+remove/reapply server status, use:
+
+```shell
+$ nomulus -e production update_server_locks \
+    --[apply | remove] {STATUS1},{STATUS2},... \
+    --client {REGISTRAR}
+    --n {DOMAIN}
+```
+
+Note that domain create restricted is intended for closed TLDs, as such, the
+validation only happens in regular domain create/update flows. Domain
+application and allocate are usually not applicable to closed TLDs because there
+is no sunrise period. Therefore no domain whitelist validation against the
+reserved lists is performed during these flows, nor are server prohibited status
+applied.